According to superintendent Ramon Cortines, the threat was "conveyed not to one school, but many schools in the district. Details talked about backpacks, other packages." He also said this was "a rare threat."

Officials will investigate the schools in the region. Cortines said he wants plant managers at the schools to call police if they spot "anything that is out of order."

The FBI and LAPD are both looking into the threat.

School board president Steve Zimmer said, "We are taking this action in abundance of caution to make sure that every child in the LA Unified School District and every employee is absolutely safe."

The mass closure affects 640,000 students from kindergarten through 12th grade in 900, as well as 187 charter schools. Los Angeles has the second largest school district in America.